Glass frits containing WO3 or MoO3 in RuO2 -based resistors

ABSTRACT

This invention is concerned with the fabrication of thick film, RuO 2  -based resistors. More specifically, this invention is directed to the formulation of glass frits for use in such resistors exhibiting temperature coefficient of resistance values of less than 100 ppm. Such glass frits consist essentially, expressed in terms of mole percent on the oxide basis, of about 32-39% PbO, 44-47% B 2  O 3 , 14-17% SiO 2 , and an effective amount up to 5% of WO 3  or MoO 3 .

BACKGROUND OF THE INVENTION

Hybrid integrated circuits are used extensively in the electronicsindustry for various purposes. One of the constituent parts of thosedevices is a thick film resistor. A thick film resistor is customarilyfabricated by screen printing a paste which contains the conductor andresistor onto a substrate such as alumina. The coated substrate is firedat a temperature appropriate to the paste composition. The pastecontains at least the following three necessary components; viz., aconducting phase, glass powder to bind the conducting phase and adhereto the substrate, and a vehicle, the latter commonly consisting of anorganic polymer and solvent, to provide the proper consistency forscreen printing.

Ruthenium dioxide (RuO₂) is widely employed in the industry as one ofthe conducting phases in thick film resistors, and glass frits havingcompositions within the PbO-B₂ O₃ -SiO₂ system have frequentlyconstituted the bond. The composition of the frit is formulated to havea coefficient of thermal expansion approximating that of the aluminasubstrate material. Unfortunately, however, the temperature coefficientof resistance demonstrated by those products has been undesirably high.Hence, the products have exhibited temperature coefficients ofresistance in excess of 100 ppm (parts per million).

Therefore, the principal objective of this invention is to develop thickfilm, RuO₂ -based resistor compositions which can be suitably applied toalumina substrates in the conventional manner, but which manifest lowtemperature coefficients of resistance, viz., less than 100 ppm and,most preferably, less than 50 ppm.

SUMMARY OF THE INVENTION

That objective can be achieved with glass frits having compositionswithin the base PbO-B₂ O₃ -SiO₂ field but wherein WO₃ or MoO₃ ispartially substituted for PbO. Upon heat treatment of the frit, crystalsof PbWO₄ or PbMoO₄ are generated in situ. Their presence is responsiblefor the improved temperature coefficient of resistance. Accordingly, anamount of WO₃ or MoO₃ effective to promote the development of thedesired crystal phase with a concomitant beneficial effect upon thetemperature coefficient of resistance will be incorporated into the baseglass composition. Whereas some desirable effect can be witnessed withsmaller substitutions, a minimum of about 1 mole percent WO₃ or MoO₃will, in general, be substituted for 1 mole percent PbO. Substitutionsof WO₃ or MoO₃ for PbO in excess of about 5 mole percent can beemployed, but such practice increases the cost of the glass, hazardsundesirable changes in the physical properties of the base glass, andprovides products exhibiting higher temperature coefficients ofresistance than where lesser amounts of WO₃ or MoO₃ are utilized. Thus,the optimum effect is experienced at about 2-4 mole percent WO₃ or MoO₃.The inventive glass compositions consist essentially, expressed in termsof mole percent on the oxide basis, of about 32-39% PbO, 44-47% B₂ O₃,14-17% SiO₂, and an effective amount of WO₃ or MoO₃ up to 5%, with thepreferred glasses containing about 2-4% WO₃ or MoO₃.

The heat treatment utilized to crystallize PbWO₄ or PbMoO₃ in situcomprises a substantive feature of the instant invention. Thus,temperatures between about 750°-950° C. are effective. Thecrystallization phenomenon is a function of time and temperature, withthe growth of crystals proceeding more rapidly at higher temperatures.For example, a like volume of crystals may be developed after anexposure of only a few minutes at the upper end of the temperature rangeas would be generated after a much longer period, e.g., up to a fewhours, at the cooler extreme of the range. Nevertheless, care must beexercised when utilizing temperatures in the hotter portion of thetemperature range since excessively long exposures may lead to there-solution of the crystals in the base glass with consequent loss ofeffect upon the temperature coefficient of resistance. It is apparent,of course, that the minimum firing temperature employed is thatsufficient to fuse the glass frit and cause adequate flow to produce asound coating or film on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a plot of TCR (temperature coefficient of resistance)demonstrated by several resistor inks versus changes in firingtemperatures.

FIG. 2 represents a plot of volume resistivity of a PbWO₄ disc versusreciprocal temperature in terms of °K.

FIG. 3 depicts a plot of resistivity exhibited by several resistor inksversus changes in firing temperature.

FIG. 4 sets out a plot of TCR displayed by several resistor inks versuschanges in firing temperature.

FIG. 5 illustrates a plot of resistivity evidenced by several resistorinks versus changes in firing temperatures.

DESCRIPTION OF PREFERRED EMBODIMENTS

Table I records a group of exemplary glass compositions, expressed interms of approximate mole percent on the oxide basis, designed toillustrate the present invention. Table IA reports the same glasses,expressed in terms of parts by weight on the oxide basis. Inasmuch asthe sum of the individual ingredients totals or closely approximates100, the values tabulated may, for all practical purposes, be deemed toreflect weight percent. The actual batch ingredients utilized may be anymaterials, either the oxide or other compounds which, when meltedtogether, will be converted into the desired oxide in the properproperties.

The batch components were compounded, ballmilled together to assist insecuring a homogeneous melt, the mixture turned into platinum crucibles,lids placed upon the crucibles, and the crucibles introduced into afurnace operating at about 1200° C. After four hours, the crucibles wereremoved from the furnace, the melt cocktail mixed to improve thehomogeneity thereof, and the melt then poured as a thin stream into acontainer of water ("drigaged") to form finely-divided glass particles.

The glass particles were further reduced in size by ball milling inmethanol employing alumina jars and cylinders. The average particle sizewas between 3-6 microns, as measured using a Coulter Counter.

A resistor ink was then prepared in the following manner. Each frittedglass was blended with about 12% by volume of RuO₂, the material usedbeing Engelhard Lot RU-158, marketed by Engelhard Minerals and ChemicalsCorporation, Iselin, N.J., containing about 75.15% Ru and having anaverage particle size of 2.6 microns. Approximately one gram of B. F.Drakenfeld 175 silk screeen oil, marketed by B. F. Drakenfeld,Washington, Pa., was added to each mixture and the resulting masshomogenized by mixing for 15 minutes in an automatic Fisher Mortargrinder, marketed by Fisher Scientific Co., Pittsburgh, Pa.

                                      TABLE I                                     __________________________________________________________________________    1      2  3  4  5  6  7  8  9  10 11 12                                       __________________________________________________________________________    SiO.sub.2                                                                         15.04                                                                            15.33                                                                            15.61                                                                            15.93                                                                            16.17                                                                            15.35                                                                            15.60                                                                            15.91                                                                            16.18                                                                            15.38                                                                            15.38                                                                            15.60                                    B.sub.2 O.sub.3                                                                   45.79                                                                            45.68                                                                            45.58                                                                            45.45                                                                            45.36                                                                            45.67                                                                            45.58                                                                            45.46                                                                            45.36                                                                            45.59                                                                            45.60                                                                            45.64                                    PbO 38.91                                                                            37.66                                                                            36.42                                                                            35.16                                                                            33.95                                                                            37.65                                                                            36.43                                                                            35.18                                                                            33.95                                                                            36.76                                                                            34.77                                                                            37.48                                    MoO.sub.3                                                                         --  1.07                                                                             2.13                                                                             3.20                                                                             4.26                                                                            -- -- -- --  1.00                                                                             2.00                                                                             0.5                                     WO.sub.3                                                                          -- -- -- -- --  1.07                                                                             2.13                                                                             3.20                                                                             4.25                                                                             1.00                                                                             2.00                                                                             0.5                                     __________________________________________________________________________

                                      TABLE IA                                    __________________________________________________________________________    1      2  3  4  5  6  7  8  9  10 11 12                                       __________________________________________________________________________    SiO.sub.2                                                                          7.1                                                                              7.3                                                                              7.5                                                                              7.7                                                                              7.9                                                                              7.2                                                                              7.4                                                                              7.5                                                                              7.6                                                                             7.3                                                                              7.3                                                                              7.4                                      B.sub.2 O.sub.3                                                                   25.0                                                                             25.1                                                                             25.3                                                                             25.5                                                                             25.6                                                                             24.9                                                                             24.9                                                                             24.9                                                                             24.9                                                                             25.0                                                                             25.2                                                                             25.1                                     PbO 68.0                                                                             66.4                                                                             64.8                                                                             63.1                                                                             61.5                                                                             65.9                                                                             63.9                                                                             61.8                                                                             59.7                                                                             64.7                                                                             61.6                                                                             66.0                                     MoO.sub.3                                                                         --  1.2                                                                              2.4                                                                              3.7                                                                              5.0                                                                             -- -- -- -- 1.1                                                                              2.3                                                                              0.6                                      WO.sub.3                                                                          -- -- -- -- --  1.9                                                                              3.9                                                                              5.8                                                                              7.8                                                                             1.8                                                                              3.7                                                                              0.9                                      __________________________________________________________________________

A silver/palladium conductor was applied to an alumina substrateutilizing Du Pont's Ag/Pd-9601 conductor paste, marketed by E. I. DuPont de Nemours & Co., Inc., Wilmington, Del., the application beingundertaken by means of a camel's hair brush. In accordance withconventional practice, the paste film was dried in the ambient air for10 minutes at 150° C. and thereafter fired in air for 10 minutes at 850°C.

The resistor inks were then silk screened onto the alumina substrates inthe customary fashion to overlap the previously-applied conductor pads(deposits). The resultant film was dried for 10 minutes in the ambientair at 150° C.

Individual samples of the alumina substrates coated with the exemplaryresistor inks were fired for 10 minutes in the ambient air at 800° C.,850° C., and 900° C. The samples were introduced directly into thefurnace at the firing temperature (plunge firing) and immediatelyremoved from the furnace to the ambient temperature environment afterthe 10-minute exposure. Plunge firing appears to yield somewhat lowertemperature coefficients of resistance than where slower firingschedules are utilized.

A silver wire, about 21/2" in length and about 0.025" in diameter, wassoldered in known manner to each conductor pad employing 60/40 resincore solder. The sheet resistivity (p) in ohms/square and thetemperature coefficients of resistance (TCR) in ppm/°C. were measuredutilizing methods conventional in the glass art. Table II illustratesthe fired thickness of each film, the resistivity of the film at roomtemperature, the resistivity normalized to a thickness of 0.001" at roomtemperature, the TCR, and the top temperature reached in evaluating theTCR.

                                      TABLE II                                    __________________________________________________________________________         Firing Fired        Normalized                                                                           TCR  Top                                      Example                                                                            Temperature                                                                          Thickness                                                                           Resistivity                                                                          Resistivity                                                                          PPM/°C.                                                                     Temperature                              __________________________________________________________________________    1    800° C.                                                                       0.0015"                                                                             3.216 × 10.sup.3                                                               4.824 × 10.sup.3                                                               294  194° C.                           1    850° C.                                                                       0.0016"                                                                             3.457 × 10.sup.3                                                               5.531 × 10.sup.3                                                               260  157° C.                           1    900° C.                                                                       0.0015"                                                                             1.840 × 10.sup.3                                                               2.76 × 10.sup.3                                                                405  144° C.                           2    800° C.                                                                       0.0015"                                                                             1.042 × 10.sup.3                                                               1.563 × 10.sup.3                                                               364  147° C.                           2    850° C.                                                                       0.0016"                                                                             0.843 × 10.sup.3                                                               1.349 × 10.sup.3                                                               365  157° C.                           2    900° C.                                                                       0.0016"                                                                             0.937 × 10.sup.3                                                               1.498 × 10.sup.3                                                               484  147° C.                           3    800° C.                                                                       0.0014"                                                                             15.64 × 10.sup.3                                                               21.896 × 10.sup.3                                                               18  157° C.                           3    850° C.                                                                       0.0018"                                                                             1.178 × 10.sup.3                                                               2.120 × 10.sup.3                                                               352  157° C.                           3    900° C.                                                                       0.0017"                                                                             0.5606 × 10.sup.3                                                              0.953 × 10.sup.3                                                               746  144° C.                           4    800° C.                                                                       0.0018"                                                                              951 × 10.sup.3                                                                1711.8 × 10.sup.3                                                              -132 154° C.                           4    850° C.                                                                       0.0022"                                                                               18 × 10.sup.3                                                                39.6 × 10.sup.3                                                                 43  154° C.                           4    900° C.                                                                       0.0014"                                                                             1.266 × 10.sup.3                                                               1.772 × 10.sup.3                                                               479  154° C.                           5    800° C.                                                                       0.0017"                                                                              ∞                                                                              --    --   --                                       5    850° C.                                                                       0.0017"                                                                             236.9 × 10.sup.3                                                               402.7 × 10.sup.3                                                               -135 154° C.                           5    900° C.                                                                       0.0017"                                                                             5.256 × 10.sup.3                                                               8.935 × 10.sup.3                                                               307  155° C.                           6    800° C.                                                                       0.0012"                                                                             1.928 × 10.sup.3                                                               2.314 × 10.sup.3                                                                97  155° C.                           6    850° C.                                                                       0.0018"                                                                             0.962 × 10.sup.3                                                               1.731 × 10.sup.3                                                               348  143° C.                           6    900° C.                                                                       0.0015"                                                                             1.530 × 10.sup.3                                                               2.295 × 10.sup.3                                                               240  135° C.                           7    800° C.                                                                       0.0014"                                                                             8.519 × 10.sup.3                                                               11.927 × 10.sup.3                                                              -42  149° C.                           7    850° C.                                                                       0.0014"                                                                             2.063 × 10.sup.3                                                               2.888 × 10.sup.3                                                               89   146° C.                           7    900° C.                                                                       0.0016"                                                                             1.456 × 10.sup.3                                                               2.330 × 10.sup.3                                                               161  138° C.                           8    800° C.                                                                       0.0014"                                                                             84.17 × 10.sup.3                                                               117.84 × 10.sup.3                                                              -75  170° C.                           8    850° C.                                                                       0.0012"                                                                             9.259 × 10.sup.3                                                               11.111 × 10.sup.3                                                              -11  170° C.                           8    900° C.                                                                       0.0015"                                                                             1.667 × 10.sup.3                                                               2.501 × 10.sup.3                                                               145  156° C.                           9    800° C.                                                                       0.0018"                                                                              ∞                                                                              --    --   --                                       9    850° C.                                                                       0.004"                                                                              81.64 × 10.sup.3                                                               326.56 × 10.sup.3                                                              -167 155° C.                           9    900° C.                                                                       0.0016"                                                                             5.933 × 10.sup.3                                                               9.493 × 10.sup.3                                                               -54  156° C.                           10   800° C.                                                                       0.0016"                                                                              ∞                                                                              --    --   --                                       10   850° C.                                                                       0.0013"                                                                             5.057 × 10.sup.3                                                               6.574 × 10.sup.3                                                               195.8                                                                              120° C.                           10   900° C.                                                                       0.0015"                                                                             3.716 × 10.sup.3                                                               5.574 × 10.sup.3                                                               896.2                                                                              120° C.                           11   800° C.                                                                       0.0015"                                                                              ∞                                                                              --    --   --                                       11   850° C.                                                                       0.0019"                                                                              ∞                                                                              --    --   --                                       11   900° C.                                                                       0.0016"                                                                             44.664 × 10.sup.3                                                              62.530 × 10.sup.3                                                              964.7                                                                              120° C.                           12   800° C.                                                                       0.0016"                                                                             3.479 × 10.sup.3                                                               5.566 × 10.sup.3                                                               319.1                                                                              120° C.                           12   850° C.                                                                       0.0011"                                                                             3.449 × 10.sup.3                                                               3.794 × 10.sup.3                                                               265.5                                                                              120° C.                           12   900° C.                                                                       0.0013"                                                                             12.787 × 10.sup.3                                                              16.623 × 10.sup.3                                                              376.0                                                                              120° C.                           __________________________________________________________________________

When Tables I and II are studied in conjunction with each other, it isquite apparent that the presence of WO₃ or MoO₃ had the effect ofsharply reducing the values of TCR. Inexplicably, however, a combinationof WO₃ and MoO₃ did not produce the same effect.

FIG. 1 illustrates the effect upon TCR as a result of varying the molepercent concentration of WO₃ in the glass frits combined with RuO₂. Theeffect of the temperature at which the resistor ink was fired upon TCRis also evident from FIG. 1. Thus, it can be observed that TCR was afunction of WO₃ concentration and resistor ink firing temperature. Forexample, as the WO₃ concentration was increased, the TCR decreased ateach firing temperature and exhibited the lowest level for eachcomposition at 800° C. As was noted above, acceptable TCR values aredeemed to be within the interval of ±100 ppm/°C., with the ideal value,obviously, being 0 ppm/°C. FIG. 1 demonstrates that a TCR of 0 ppm/°C.can be achieved with this particular frit-RuO₂ composition at an optimumfiring schedule.

X-ray diffraction (XRD) analyses identified the presence of PbWO₄crystals in the fired resistors that contained WO₃ in the fritcompositions and indicated that the TCR values decreased with anincrease in the quantity of the PbWO₄ phase. Furthermore, XRD analysesdemonstrated that the concentration of PbWO₄ crystals decreased as thefiring temperatures were increased, thereby providing an explanation forthe observed increase in TCR with higher firing temperatures.

The melting point of PbWO₄ is reported in the literature to be 1123° C.,well above the firing temperatures employed to fuse the inventiveresistor inks. Differential scanning calorimetry (DSC) analysis ofExample 9 indicated an exotherm at 575° C. and XRD analysis identifiedPbWO₄ crystals when pressed discs of that glass frit were fired to thattemperature in the following fashion:

Room temperature to 450° C. at a rate of 100° C./hour;

Heat from 450° C. to 575° C. at a rate of 50° C./hour;

Hold at 575° C. for 1 hour;

Cool to room temperature at furnace rate.

These data evidence that the PbWO₄ phase crystallizes from the glass ata relatively low temperature and begins to redissolve at the highertemperatures required to fuse the resistor inks. Through optimization ofthe glass frit composition, the PbWO₄ phase may be maintainable athigher firing temperatures.

The low TCR values exhibited in commercially-available, high quality,thick film resistors is commonly achieved through what have been termedcompensation techniques. For example, in the case of PdO-Ag thick filmresistors, the positive TCR of the Ag-Pd is compensated by the negativeTCR of PdO. The small TCR values of resistors prepared from inks basedupon the use of resinates can be accounted for in a like manner. Toillustrate, when the Au-Rh resinate inks are fired, the positive TCR ofthe gold particles in the thick film resistor is offset by the negativevalue of lead rhodate formed by a reaction between the rhodium compoundand the lead silicate glass during the firing process.

To obtain a better understanding of the mechanism providing the low TCRlevels in the inventive inks, the volume resistivity of a disc formedfrom commercially available PbWO₄ was investigated. The disc was shapedby pressing a body from particulate PbWO₄, sintering the shape by firingto 800° C. at 100° C./hour, and then cooling to room temperature atfurnace rate. FIG. 2 represents a plot of volume resistivity in terms ofreciprocal temperature (°K.). As can be observed, the material issemiconducting with a negative TCR. The activation energy for conductionis 0.79 ev. A potential of 10 volts D.C. was applied to the specimen forseveral minutes at 530° C. No change in resistivity with time was noted,indicating the material to be electronically, rather than ionically,conducting.

FIG. 3 compares the resistivities demonstrated by Examples 1 and 6-9. Itis evident that, at the lower firing temperatures, the resistivities arehigher than the control glass (Example 1). As the firing temperature wasincreased, however, the resistivities became lower and all compositions,with the exception of Example 9, had substantially equivalentresistivities at the 900° C. firing level. The higher resistivitiesexhibited at the lower firing temperatures are believed to be the resultof increased glass viscosity resulting from the crystallization ofPbWO₄. It has been postulated that the increase in glass viscosityreduces flow of the frit and increases the conductive path between theconductive species. Optimization of the frit composition can lead toreduced resistivity at the lower fusing temperatures.

FIGS. 4 and 5 illustrate the TCR and sheet resistivity, respectively, ofExamples 2-5, the MoO₃ analogs of the WO₃ glasses. The 1 mole percentMoO₃ glass (Example 2) showed an unexplained increase in TCR, whencompared with Example 1, the control glass, at all temperatures. TheMoO₃ -containing glasses demonstrated a much steeper rise in TCR withhigher firing temperatures than the WO₃ -containing glasses. Moreover,Examples 3 and 4 exhibited higher values of TCR at the 900° C. firingtemperature than Example 1. No firm rationale has been developed forunderstanding those phenomena, but it has been theorized that thesharper increase in TCR may be the result of the PbMoO₄ crystals(determined by XRD analysis to be present in Examples 2-5) redissolvingin the glass phase at a more rapid rate than the PbWO₄ crystals at thetemperatures evaluated.

A comparison of the resistivity values of the WO₃ -containing glasses(Examples 6-9) with those of the MoO₃ -containing glasses (Examples 2-5)can be made by examining FIGS. 3 and 5. It is believed evident that theresistivities are quite similar.

I claim:
 1. Glass frit compositions for use in thick film, RuO₂ -basedresistor inks suitable for application to alumina substrates exhibitingcoefficients of thermal expansion approximating that of the aluminasubstrate material and temperature coefficient of resistance values ofless than 100 ppm which consist essentially, expressed in terms of molepercent on the oxide basis, of about 32-39% PbO, 44-47% B₂ O₃, 14-17%SiO₂, and an effective amount of WO₃ or MoO₃ up to 5%.
 2. Glass fritcompositions according to claim 1 wherein said WO₃ or MoO₃ contentranges about 2-4%.